When manufacturing absorbent articles such as diapers, sanitary napkins, incontinence protection article, etc., it is known to use a continuous conveyer belt onto which at least a part of the absorbent article is assembled. The sanitary napkin may comprise a topsheet, a backsheet and an absorbent body positioned therebetween. In the assembly line the absorbent body is positioned onto the conveyer belt and the topsheet is positioned onto the absorbent body. In the assembly line, the topsheet or the backsheet is in the form of a continuous web that is cut after being positioned onto the absorbent body.
When the first web comprises a pattern the pattern has a first centre line that shall be positioned in line with a second centre line comprised in the absorbent body. It is important that the first centre line is an optic centre line, i.e. the first centre line shall be a centre line according to the eye of an observer. When the first web comprises an elastic material the shape and form of the first web may vary due to material variations and/or strains in the material caused, for example, by machine parts in the manufacturing plant.
The machine parts may be rollers and stretchers, etc. arranged to bring the first web in a machine direction. When the first web changes form and shape the first centre line changes position, and as a consequence the position of the first centre line in relation to the second centre line is changed.
It is common to use the first web as a topsheet and that the topsheet comprises two side portions of a first material and a centre portion of a second material therebetween. The centre portion is attached to the side portions by, for example, welding, gluing, mechanical bonding etc. and the attachment process forms seams that become part of or form the pattern of the first web. In the assembly line the seams run in the machine direction, i.e. in the feeding direction of the first and second webs. The position of the seams may vary somewhat with respect to the lateral side edges of the first web due to different factors such as material variations and external forces acting on the first web. The shifting of the position of the seams affects the position of the first centre line, i.e. the optic centre line. Each lateral side edge of the first web also has an extension in the machine direction and since the seams vary with respect to the lateral side edges, it is not a sufficiently good solution to calculate the first centre line from the position of one or both of the side edges since this obvious gives a wrong position of the first centre line in the first web.
US 2004/0028268 teaches a lamination process where a first web is laminated to a second web forming a composite web. The process comprises the use of a camera that periodically takes pictures of the composite web. The pictures are converted into electrical signals and are compared to a number of selected parameters in order to determine if the first web is properly aligned with the second web. A problem with the process according to US 2004/0028268 is that the system detects a fault only after the composite web has been formed. The system allows for adjustment so that the fault is corrected, but the faulty part has to be rejected or accepted.
There is thus a need to control the first web in the lateral direction, i.e. in a direction being essentially perpendicular to the machine direction, so that the first centre line becomes aligned with the second centre line before assembly. The second centre line may be the centre line of a continuous web but may be the centre line of a broken web comprising a number of pieces of material arranged onto a conveyer belt.
Therefore, there is a need for an adaptive assembly line for detecting the first centre line and controlling the alignment of the first centre line of the first web to the second centre line of the second web, before the first web is positioned onto the second web.